This invention relates to vortex flow sensors for measuring the flow velocity or the volumetric flow rate of a fluid flowing through a measuring tube, comprising a bluff body mounted along a diameter of the measuring tube and serving to generate Kxc3xa1rmxc3xa1n vortices.
During the operation of such a vortex flow sensor, a Kxc3xa1rmxc3xa1n vortex street is formed downstream of the bluff body. Its pressure fluctuations are converted by a vortex sensing element into an electric signal whose frequency is proportional to the volumetric flow rate.
EP-A 841 545, corresponding to U.S. patent application Ser. No. 08/953,229, filed Oct. 17, 1997, discloses a vortex flow sensor for measuring the flow velocity and/or the volumetric flow rate of a fluid flowing through a measuring tube, comprising:
a bluff body serving to generate Kxc3xa1rmxc3xa1n vortices which is disposed along a diameter of the measuring tube and connected with the measuring tube at at least one fixing point; and
a capacitive vortex sensing element responsive to vortexinduced pressure fluctuations which is inserted in a wall bore of the measuring tube downstream of the bluff body, sealing off the bore toward the external surface of the measuring tube, with the center of the bore and the center of the fixing point lying on a straight surface line of the measuring tube, and which comprises:
a diaphragm covering the bore and having a first surface facing toward the fluid and a second surface remote from the fluid;
a rigid, thin sensor vane which is attached to the first surface, is shorter than the diameter of the measuring tube, and has flat major surfaces which are in alignment with the straight surface line of the measuring tube; and
an electrode arrangement attached to the second surface.
EP-A 841 545 further discloses a vortex flow sensor for measuring the flow velocity and/or the volumetric flow rate of a fluid flowing through a measuring tube, comprising:
a bluff body serving to generate Kxc3xa1rmxc3xa1n vortices which is disposed along a diameter of the measuring tube, is connected with the measuring tube at at least one fixing point, and has a main bore, extending in the direction of said diameter and through the measuring tube, and at least one secondary bore, which connects the main bore with the fluid; and
a capacitive vortex sensing element responsive to vortex-induced pressure fluctuations and inserted in the main bore, sealing of f the external surface of the measuring tube from the fluid, said capacitive vortex sensing element comprising:
a diaphragm covering the measuring-tube-side end of the main bore and having a first surface facing toward the fluid and a second surface remote from the fluid;
a rigid sensor vane attached to the first surface; and
an electrode arrangement attached to the second surface.
As the two vortex flow sensors disclosed in EP-A 841 545 show, the vortex sensing element can be disposed either in the bluff body or in the bore in the wall of the measuring tube. Although such vortex flow sensors have proved to be effective in practice, it is desirable to use piezoelectric sensing elements instead of capacitive sensing elements. The use of piezoelectric elements in vortex flow sensors is in the prior art.
U.S. Pat. No. 4,248,098, for example, discloses a vortex flow sensor for measuring the flow velocity and/or the volumetric flow rate of a fluid flowing through a measuring tube, comprising:
a bluff body serving to generate Kxc3xa1rmxc3xa1n vortices which is disposed along a diameter of the measuring tube and is permanently connected with the measuring tube at two fixing points; and
a vortex sensing element in the form of at least one piezoelectric element responsive to vortex-induced mechanical stresses in the bluff body, said vortex sensing element being disposed
either in a recess of the bluff body provided in the area of one of the fixing points
or in a recess provided in a prolongation of the bluff body extending toward the outside of the measuring tube, with
either two individual piezoelectric elements being provided each having one electrode on two opposite surfaces
or a single piezoelectric element being provided having two electrically separated electrodes on a first surface and a common electrode on an opposite second surface.
Furthermore, JP-A 58-160 813 discloses a vortex flow sensor for measuring the flow velocity and/or the volumetric flow rate of a fluid flowing through a measuring tube, comprising:
a bluff body serving to generate Kxc3xa1rmxc3xa1n vortices which is disposed along a diameter of the measuring tube and is permanently connected with the measuring tube at only one fixing point; and
a vortex sensing element responsive to vortex-induced mechanical stresses in the bluff body, said vortex sensing element being disposed
either in a recess of the bluff body provided in the area of the fixing point toward the outside of the measuring tube, with
either two individual piezoelectric elements being provided each having one electrode on two opposite surfaces
or a single piezoelectric element being provided having two electrically separated electrodes on a first surface and a common electrode on an opposite second surface.
EP-A 319 424 discloses a vortex flow sensor for measuring the flow velocity and/or the volumetric flow rate of a fluid flowing through a measuring tube, comprising:
a bluff body serving to generate Kxc3xa1rmxc3xa1n vortices which is disposed along a diameter of the measuring tube and is permanently connected with the measuring tube at two fixing points; and
a vortex sensing element responsive to vortex-induced pressure fluctuations and consisting of two piezoelectric elements which
are disposed on a surface of a diaphragm of the bluff body remote from the fluid, said diaphragm being provided at one of the fixing points.
Finally, U.S. Pat. No. 5,197,336 discloses a vortex flow sensor for measuring the flow velocity and/or the volumetric flow rate of a fluid flowing through a measuring tube, comprising:
a bluff body serving to generate Kxc3xa1rmxc3xa1n vortices which is disposed along a diameter of the measuring tube and is permanently connected with the measuring tube at two fixing points; and
a vortex sensing element responsive to vortex-induced pressure fluctuations which
extends downstream of and near and parallel to the bluff body and is permanently connected with the measuring tube at two opposite fixing points, and
is provided on a cylindrical prolongation extending toward the outside of the measuring tube, said cylindrical prolongation supporting a concentric piezoelectric element.
It is an object of the invention to modify the prior-art designs of vortex flow sensors with piezoelectric elements in such a way that the vortex sensing element can be assembled from individual components in a simple manner, so that a faulty piezoelectric element can be easily replaced. Furthermore, it is to be possible to make the vortex sensing element largely insensitive to external vibrations.
To attain these objects, a first variant of the invention provides a vortex flow sensor for measuring the flow velocity and/or the volumetric flow rate of a fluid flowing through a measuring tube, comprising:
a bluff body serving to generate Kxc3xa1rmxc3xa1n vortices which is disposed along a diameter of the measuring tube and is connected with the measuring tube at at least one fixing point; and
a vortex sensing element responsive to vortex-induced pressure fluctuations and inserted in a wall bore of the measuring tube downstream of the bluff body, sealing off the bore toward the external surface of the measuring tube, with the center of the bore and the center of the fixing point lying on a straight surface line of the measuring tube, said vortex sensing element having the following features:
a diaphragm covering the bore and having a first surface facing toward the fluid and a second surface remote from the fluid;
a rigid, thin sensor vane which is attached to the first surface of the diaphragm, is shorter than the diameter at the measuring tube, and has flat major surfaces which are in alignment with the straight surface line of the measuring tube; and
a piezoelectric element mechanically coupled to the second surface of the diaphragm.
To attain the above objects, a second variant of the invention provides a vortex flow sensor for measuring the flow velocity and/or the volumetric flow rate of a fluidf lowing through a measuring tube, comprising:
a bluff body serving to generate Kxc3xa1rmxc3xa1n vortices which is disposed along a diameter of the measuring tube, is connected with the measuring tube at at least one fixing point, and has a main bore extending in the direction of said diameter and through the measuring tube as well as a secondary bore connecting the main bore with the fluid; and
a vortex sensing element responsive to vortex-induced pressure fluctuations which is inserted in the main bore, sealing off the external surface of the measuring tube from the fluid, and has the following features:
a diaphragm covering the measuring-tube-side end of the main bore and having a first surface facing toward the fluid and a second surface remote from the fluid;
a rigid sensor sleeve attached to the first surface of the diaphragm; and
a piezoelectric element mechanically coupled to the second surface of the diaphragm.
In a first preferred embodiment of the first or second variant of the invention, the piezoelectric element
is attached to the second surface of the diaphragm,
has a base electrode on a surface facing toward the diaphragm, and
has a first electrode and a second electrode on a surface remote from the diaphragm which are symmetrical with respect to a median plane of the sensor vane.
In a second preferred embodiment of the first or second variant of the invention,
a first end of a flexible beam is fixed at the center of the second surface of the diaphragm, and a second end of the flexible beam is fixed to a holder rigidly connected with the wall of the measuring tube;
a first piezoelectric element is bonded lengthwise to a first surface of the flexible beam; and
a second piezoelectric element is bonded lengthwise to a surface of the flexible beam opposite the first surface. In a third preferred embodiment of the first or second variant of the invention,
a first end of a flexible beam is fixed at the center of the second surface of the diaphragm, and a second end of the flexible beam is fixed to a holder rigidly connected with the wall of the measuring tube;
a first piezoelectric element is mechanically fixed lengthwise to a first surface of the flexible beam; and
a second piezoelectric element is mechanically fixed lengthwise to a surface of the flexible beam opposite the first surface.
According to a first preferred development of the first embodiment of the invention,
the diaphragm comprises a retaining ring,
to one end of which the diaphragm is fixed, and
which is thicker than the diaphragm;
an intermediate ring with an inside diameter, supported on the retaining ring and screwed to the measuring tube, forces the diaphragm against the external surface of the measuring tube, with a sealing ring interposed between the diaphragm and the measuring tube;
on the surface of the piezoelectric element remote from the diaphragm, a contact-making body having a first surface facing toward the piezoelectric element and a second surface remote from the piezoelectric element is provided which
is made of insulating material, has, on the surface facing toward the piezoelectric element, a first contact area contacting the first electrode and a second contact area contacting the second electrode, and
has, on the surface remote from the piezoelectric element, a third contact area electrically connected to the first contact area and a fourth contact area electrically connected to the second contact area;
a thrust ring having a diameter slightly smaller than the inside diameter of the intermediate ring is provided on the contact-making body; and
a clamping plate supported on the thrust ring and screwed to the intermediate ring forces the thrust ring against the contact-making body, the contact-making body against the piezoelectric element, and the piezoelectric element against the diaphragm.
According to a second preferred development of the first embodiment of the invention,
the piezoelectric element and the contact-making body are annular, each having a central hole; and
on the surface of the diaphragm remote from the fluid, a compensating body extending into the central holes is provided,
with the areal moment of inertia of the sensor vane at the surface of the diaphragm facing toward the fluid being approximately equal to the areal moment of inertia of the compensating body at the surface of the diaphragm remote from the fluid.
According to a third preferred development of the first embodiment of the invention,
the piezoelectric element and the contact-making body are annular, each having a central hole; and
on the surface of the diaphragm remote from the fluid, a compensating body extending into the central holes is provided,
with the areal moment of inertia of the sensor sleeve at the surface of the diaphragm facing toward the fluid being approximately equal to the areal moment of inertia of the compensating body at the surface of the diaphragm remote from the fluid.
One advantage of the invention is that the vortex sensing element of the vortex flow sensor is assembled from individual components in a simple manner, so that a faulty piezoelectric element can be easily replaced. Furthermore, the vortex sensing element can be made largely insensitive to vibrations acting from outside.